1. Field of the Invention
The present invention relates to a semiconductor device including thin-film semiconductor layers which are different from one another in crystallinity, a substrate of the device, and a method of manufacturing the device and substrate.
2. Description of the Related Art
In thin-film semiconductor devices such as a crystalline thin-film semiconductor transistor (TFT), as well known, insulating thin films of semiconductor materials, such as silicon, are formed apart from one another on a substrate formed of an insulating material such as alkali glass, non-alkali glass, or quartz glass. A source region, a drain region, and a channel region disposed between the regions are formed on each insulating semiconductor thin film, and a gate electrode is disposed on each channel region via a gate insulating film to constitute TFT.
As shown in FIGS. 11A to 11E, the above-described thin-film semiconductor device is usually manufactured as follows.
A thin film 202 formed of a non-single-crystalline semiconductor (e.g., amorphous or polycrystalline silicon) is formed on a substrate 201 of an insulating material (e.g., glass) via an underlying layer (FIG. 11A). The whole surface of the thin film 202 is irradiated with an energy beam (e.g., excimer laser light) 203 to anneal/process this thin film 202, and the semiconductor in the thin film 202 is crystallized or recrystallized. That is, crystallinity of the semiconductor thin film 202 is improved (this annealed/processed semiconductor thin film is shown by reference numeral 204 in FIG. 11B). Here, crystallinity means “a ratio (percentage) of a mass of a crystal portion of a structure including the crystal portion and an amorphous portion to that of a whole specimen. Additionally, the amorphous portion can be removed or separated by etching, or can be evaluated with Raman spectrometry, and the like in the structure irradiated with the crystal portion and the amorphous portion microscopically exist in a mixed manner. That is, a single crystalline structure has a crystallinity of 1, and an amorphous structure has a crystallinity of 0.” The semiconductor thin film 204 crystallized or recrystallized, that is, improved in the crystallinity is processed into a plurality of isolating semi-conductor thin films 205 separated from one another by photolithography, etching, and the like. FIG. 11C shows only one insulating semiconductor thin film 205. Thereafter, a gate insulating film 206 formed of material such as silicon oxide (SiO2) is formed on the substrate 201 including the surface of the insulating semiconductor thin film 205 (FIG. 11C). Moreover, a gate electrode 207 is formed on the gate insulating film 206, and next the gate electrode 207 is used as a mask to selectively implant impurity ions such as phosphor into the insulating semiconductor thin film 205 (FIG. 11D). As a result, a source region 209 and a drain region 210 doped with impurities, and a channel region 211 positioned between these regions are formed in the semiconductor thin film. Next, contact holes are formed in the portions of the gate insulating film 206 on the source region 209 and drain region 210. A source electrode 212 and a drain electrode 213 are formed so as to be electrically connected to the source region 209 and drain region 210 via the contact holes in the gate insulating film 206 (FIG. 11E). The TFT is completed in this manner.
The above-described anneal process for crystallizing or recrystallizing the non-single-crystalline semiconductor is an especially-important process in this manufacturing process of the TFT, because a crystalline state of the semiconductor produced by this process influences characteristics of the TFT. A representative example of the anneal process will be described with reference to FIG. 12.
In FIG. 12, reference numeral 301 denotes an excimer laser light source, and a laser light 301a emitted from this source is incident upon a beam homogenizer 302. As a result, the beam homogenizer 302 shapes the laser light 301a so as to obtain a uniform light intensity and to obtain a rectangular section (e.g., 150 mm×200 μm), and the light is incident onto the non-single-crystalline semiconductor thin film 202 on the substrate 201. As a result, the rectangular region irradiated with the laser light is crystallized. In this operation, when the substrate 201 is intermittently moved in a direction shown by an arrow, that is, in a direction crossing at right angles to a longitudinal direction of a slit-shaped section of the laser light, substantially the whole surface of the semiconductor thin film 202 is uniformly irradiated with the laser light, and crystallized.
In the subsequent process, as described with reference to FIGS. 11B to 11E, the crystallized semiconductor thin film is separated into insulating portions. For display devices such as a liquid crystal display, a plurality of TFTs functioning as switching devices with respect to pixels are formed on the same substrate to form an active matrix type circuit.
When an actual liquid crystal display is constituted, the TFT for the pixel is disposed for each pixel, and therefore needs to be formed on a center part (region denoted by 201a of FIG. 13) of the substrate 201. On the other hand, the TFT for a driving circuit constituting a driver circuit which drives the liquid crystal display is disposed on a vacant region of the substrate 201, that is, on a peripheral portion of the substrate 201 (region denoted by 201b of FIG. 13). Moreover, since the TFT for the pixel is different from the TFT for the driving circuit in characteristics required for the both, it is general to form the transistors in different processes. For example, since the TFT for the pixel requires a comparatively high withstand voltage, it is desirable to form the amorphous semiconductor thin film or the polycrystalline semiconductor thin film as a basis (channel). Since the TFT for the driving circuit requires fast switching characteristics, it is desirable to form the polycrystalline or single crystalline semiconductor thin film large in mobility as the basis. Therefore, both types of TFTs need to be manufactured in the separate processes, and this has a disadvantage that the manufacturing is laborious.
Moreover, when the TFT for the driving circuit is disposed in the peripheral portion of the substrate, the TFT for the pixel has to be connected via a long data line, and there are also disadvantages that a signal loss is generated and that an operation speed is delayed.